1. Field of the Invention
This invention relates to a wiring substrate, particularly to a wiring substrate including terminal pads for bonding solder bumps, and a process for manufacturing the wiring substrate.
2. Description of the Related Art
The following is a list of the related art:
Patent Literature 1—Japanese Patent Office Official Gazette JP-A-2002-4098;
Non-Patent Literature 1—Hitachi Metals Technical Report: “Evaluation of Lead-Free Solder Balls Produced by Uniform Droplet Spray Method”, vol. 18 (2002), page 43;
Non-Patent Literature 2—Toyota Central R & D Labs. R & D Review: “Development of Highly Reliable Sn—Ag Lead-Free Solder Alloy”, vol. 35, No. 2 (2000), page 39.
A package used for enclosing and connecting an IC or LSI chip therein has a type of wiring substrate comprising a wiring laminate formed by laminating dielectric insulating layers made of a high-molecular polymer or ceramic and inner wiring metal conductors so that the inner wiring conductors are sandwiched and insulated from each other by the dielectric layers. Metal terminal pads are disposed on an outer surface of the wiring substrate so as connect with an IC-flip chip or a motherboard by soldering in a ball-grid array (BGA) or pin-grid array (PGA) formation. Most of these terminal pads are connected to the inner wiring conductors through via-conductors penetrating at least one of dielectric layers. The inner wiring conductors and via-conductors are normally formed from metal having good electrical conductivity such as copper, and the metal terminal pads also comprise such metal.
The terminal pads formed from copper are plated with nickel and further plated with gold so as to prevent oxidation of copper and to improve their bonding and wetting with solder bumps to be formed thereon for contacting the IC chip or motherboard.
An electroless-nickel plating bath that is conventionally employed for plating nickel for the terminal pads contains a phosphoric acid compound such as sodium hypophosphite as a reducing agent and further necessarily contains phosphorus so that a nickel plating layer resultantly formed by electroless-nickel plating contains as much as 4 to 8% by mass of phosphorus. When solder that is placed on the terminal pad melts, an outermost gold plating layer diffuses and is incorporated into the solder, such that the underlying electroless nickel plating layer contacts the solder.
Relating to this, it is conventionally known that when the electroless-nickel plating layer contains P (phosphorus), a brittle phosphorus-rich nickel layer is formed at an interface between a lead-free solder and the electroless-nickel plating layer, as disclosed in Non-Patent Literature 1. Further, it is conventionally known from Non-Patent Literature 2 that P lowers flowability of the lead-free solder comprising mainly tin and decreases the bonding strength of the solder with the nickel plating layer.
It has often been pointed out that the strength of a solder joint formed on an electroless-nickel plating layer containing phosphorus is lowered due to boundary separation when a wiring substrate having such a solder joint is distorted or is subject to shock such as a dropping impact. Since a Ni—Sn alloy layer is formed by reaction of Ni (nickel) in the plating layer and Sn (tin) of the solder, the solder is likely to lose ductility, possibly reducing its bonding strength.
Moreover, a problem of environmental pollution has recently prompted a need for replacing Sn—Pb eutectic solders by Pb-free solders not containing harmful lead. Pb-free solders are mainly composed of tin (Sn) and they are eutectic solders containing auxiliary constituents such as silver, copper, zinc, and bismuth, and not containing Pb (lead). Although solders that contain a small amount of Pb are available today, their auxiliary constituents other than Sn are mainly those selected from the elements mentioned above. The lead-free solder is inferior in ductility, compared to the Sn—Pb eutectic solder, and is more likely to form a solder joint having a problem such as boundary separation.
As reported in Non-Patent Literature 1, it is considered that formation of the phosphorus-rich layer is a direct cause of boundary separation of the solder joint. In fact, the boundary separation of any solder joint is remarkably suppressed by employing phosphorus-free nickel plating such as Ni (nickel)—B (boron) electroless-plating or nickel-electroplating that does not rely on phosphorus.
In a process of electroplating nickel for the terminal pads, however, it is necessary to form a very complicated pattern for tie bars that are to be electrically connected to the terminal pads formed on a pad-forming area of the wiring substrate. This electroplating process makes it difficult to reduce the distance between the terminal pads beyond a certain extent, as it is necessary to ensure space for the insertion of tie bars between the terminal pads. Consequently, the, pad forming area is increased, causing additional design restrictions.
The Ni—B electroless-nickel plating also presents a problem. Since it employs a hydrogenated boron compound as a reducing agent, the Ni—B electroless nickel plating generates, during a reducing reaction for nickel deposition, a large amount of hydrogen gas. The hydrogen gas is easily incorporated into an electroless nickel plating layer, resulting in defects such as foaming and swelling of the nickel plating layer.